Such a process is known, for example, from EP 2 743 094 A1. Applying synthetic resin to the upper side of the carrier board is intended to make it possible to use a decor paper that is impregnated on only one side, because the liquid resin applied to the carrier board solubilises the pre-dried impregnating agent on the back side of the decor paper, so that the decor paper adheres more quickly to the carrier board. Before the decor paper is applied, the synthetic resin layer applied in liquid form on the upper side must be dried.
In the wooden composite board disclosed in DE 10 2007 012 236 B4, a carrier layer of paper is first applied to the upper side. A decorative layer is applied to this carrier layer, and then a sealing lacquer coat is applied to the decorative layer. The lacquer coat is then hardened completely by means of electron beams and a structure is embossed in the lacquer coat.
Individual panels are then cut out of a large-sized laminate board so finished, which panels can be used as a floor, wall or ceiling covering. The panels can be provided with a tongue-and-groove connection on their side edges. A problem with laminates having a paper structure is delamination, which is to be encountered wherever materials are joined together layerwise with the addition of binder, heat and pressure. Delamination can in many cases be seen externally as blisters, or the paper layer in a panel becomes detached from the board at the edges.
In order to rule out delamination, there has occasionally been a move towards coating boards directly.
DE 197 51 115 A1 discloses a method of coating a panel, in which at least one coloured layer is applied to the surface by means of a printing method, in particular by means of screen printing. The surface can thereby be untreated, ground or pretreated, in particular lacquered. The applied coloured layer can finally be covered by coating with clear lacquer.
In DE 10 2004 026 739 A1 there is described a method in which an undercoat is applied by means of a roller to the upper side of the wooden composite board and dried. A first and second lacquer layer which hardens under the action of UV light is then applied to the dried undercoat, hardening not being completed. Finally, a UV-hardenable top-coat lacquer is applied to the first two lacquer layers, and the topmost lacquer layer is then plastically deformed. Direct printing is very expensive and must be carried out very carefully in order to avoid inclusions in the print structure. In addition, it is important that each individual layer is dried sufficiently before the next layer is applied in liquid form, in order to prevent colours from merging.
When direct printing technology is used, that is to say when the individual layers are printed directly onto a carrier board, there is ultimately a reduction in thickness of the finished laminate panels compared to panels manufactured in the conventional manner owing to the omission of the paper plies. This leads to problems in continuous production, when the large-sized coated laminate boards are then divided up to produce the panels. In order to ensure that individual panels do not fail to meet DIN 13329 because they are undersized in terms of thickness, rigorous quality assurance must be implemented, which on the one hand slows down manufacture and on the other hand increases the production costs further. Ultimately, it also causes irritation for the retailer and/or end user, since the stacking height of direct-coated boards is visibly different from that of boards coated with paper plies.
In the known board manufacture, HDF boards or MDF boards are conventionally used as the carrier board, the upper side of which is ground off by about 0.3 mm. A press skin, also called a press patina or rotting layer, forms on the upper side and the underside of the carrier board. This press skin forms during pressing of the pressed fibre mat and is produced by the hot surface of the pressing plates or belts of the press. The press skin has a thickness of about 0.3 mm. Since the press skin is ground off completely and about 0.1 mm of the core material is ground off in order to reduce the surface roughness on the upper side of the board prior to further coating, the boards must be manufactured thicker by a corresponding grinding allowance, which adversely affects the production costs. In order to produce a floor laminate having a thickness of 6.0 mm which conforms to standards and is produced by direct printing technology, the carrier board must have at least 0.61 mm.
In EP 2 236 313, for example, it is described that the press skin must be ground off because the heat input in the region thereof during hot pressing is so high that the adhesive hardens too quickly, so that glue bridges break at least partly and make the applied layer sensitive. This breaking of the glue bridges makes finished panels susceptible to lifting of the decorative and abrasion-resistant layers applied to the carrier board. This delamination, which can occur even under normal loading and out of the order of magnitude typical for wooden composite materials due to climate fluctuations, is not tolerated by the customer.